1. Field of the Invention
This invention relates to the field of infrared sensing, and more particularly, to a method and apparatus for scanning thermal images.
2. Description of Related Art
Elemental infrared detectors are often used in conjunction with missiles and night vision systems to sense the presence of electromagnetic radiation having a wavelength of 1-15 .mu.m. These detectors often operate on the principle of photoconductivity, in which infrared radiation changes the electrical conductivity of the material upon which the radiation is incident. Such detectors are often fabricated from mercury-cadmium-telluride, though other materials such as CdTe and CdSe are also used.
While an array of elemental infrared detectors may be used in an elemental system in which the detectors sense the average energy generated by an object space, they may also be used in thermal imaging systems. In one such imaging system using a charge coupled device ("CCD"), the elemental detectors produce free charge carriers which are then injected into the CCD structure and are processed by using time delay integration and parallel-to-serial scan conversion. In real time thermal imaging systems such as forward looking infrared ("FLIR") imaging sensors, moving mirrors are used to scan radiation emitted by the object space across an array of elemental detectors, the temporal outputs of which are a two-dimensional representation of the thermal emission from the object space.
The optical system of an imaging sensor projects a real image of the scene (or object space) upon the plane (usually referred to as the focal plane) containing the detector array sensitive surface. The array may be two dimensional, with the corner elements viewing the corners of the desired image or sensor field-of-view ("FOV"). The array may be essentially one dimensional (usually referred to as a linear array, perhaps with multiple rows) where the end elements define two edges of the FOV but the narrow dimension of the array is much smaller than the other image dimension and the image must be moved (or scanned) in a direction normal to the long dimension of the array in order for the linear array to cover the desired FOV. The array may also be essentially a point in the sense that both dimensions of the detective array are much smaller than the desired image or FOV, and the image must be scanned in two directions across the detector in order for the detector to cover the desired FOV. The relative movement of a linear array from one edge of the FOV to the opposite edge, or of a point detector from one corner of the FOV to the opposite corner, generates a field of image infomation. The two dimensional detective array, used in what is referred to as a "staring sensor", generates a field of information without relative motion between the detective array and image. In all three cases, the individual elements of the detective array will have non-zero area and dimensions, and the detective array will cover some part of the total image area during each field.
In general, there will be some space between individual elements, the area swept out by the detector elements in one field will be less than the total area of the image, and some image information may be lost. For this reason many sensors operate in the interlace mode. Consider, for example, a linear array with adjacent detector elements separated by spaces equal to the detector height, where height is the dimension parallel to the long dimension of the array. In one field this array would cover or sweep out one-half the image area. In the interlace mode of operation, the image would be shifted one element height in the direction parallel to the array length and a second field generated. The combination of two fields, which together cover the desired image, is generally called a frame. The same approach may also be needed by and applied to two dimensional arrays (staring sensors) and point arrays (used in what are generally referred to as serial scanners). In the example given, the interlace ratio is 2:1 since it takes two fields to generate one complete image (or frame). Interlaced operation is also used to reduce signal band width.
When each field covers exactly half the image, there is no overlapping of fields, and the sensor is said to have zero overscan (usually given as a percentage). Some overscan may be desireable. Returning to the previous example of 2:1 interlace, increasing the detector height while keeping everything else constant allows the fields to overlap. The increased detector size produces increased spatial filtering. As another example, keeping the detector geometry constant but doubling the interlace ratio produces 100% overscan and can reduce image artifacts due to aliasing. In both these cases, the centers of different detective elements do not sample the same image point (for a staring sensor) or image line (for a linear array).
When used in conjunction with certain imaging systems, the output from each elemental detector is often coupled to the amplifying electronics through an A.C.-coupling circuit. Such A.C.-coupling circuits generally provide three advantages when used in imaging systems. The first of these advantages is that good contrast rendition of the object space requires background subtraction, which can generally be approximated by using an A.C.-coupling circuit. Secondly, the D.C. biasing potential supplied to an elemental detector can be removed by the coupling circuit so that the biasing potential will not influence the subsequent processing of the detector output. Finally, an A.C.-coupling circuit is able to minimize the effects of detector l/f noise on the processing electronics.
Because the implementation of the A.C.-coupling circuit often requires an RC high-pass network, the circuit will generate a zero output voltage when a D.C. signal representing the average thermal intensity of the object space is produced by the elemental detectors. While the elemental detectors could therefore sense variations in thermal intensity of the object space, the average intensity could not be determined without some means for restoring the D.C. portion of the detector output.
To restore this D.C. portion of the detector output after the output had passed through an A.C.-coupling circuit, the imaging sensor was often designed to scan a thermal reference source during an inactive portion of the scan cycle. The thermal reference source would often comprise a passive source such as a field stop or an active source such as a heated strip. When the thermal emission from the thermal reference source was received by a detector, the last coupling capacitor output was shorted to ground. By shorting the coupling capacitor in this manner, the capacitor would rapidly charge to a D.C. value equal to the signal produced by the detector upon receipt of the thermal emission of the thermal reference source. When the detector reached the active portion of the scan cycle, the circuit resumed normal operation allowing passage of the signal variation around the thermal reference signal voltage.
In addition, to compensate for differences in responsivities (i.e., the rms signal voltage generated by a detector per unit rms radiant power incident upon the detector) between the detector channels (i.e., the detector together with its coupling and amplifying electronics), it was often necessary to use a second thermal reference source. At different times during the inactive portion of the scan cycle, each elemental detector would receive thermal emissions from each of the thermal reference sources. Because the thermal reference sources emitted different intensities of infrared radiation, the responsivities of the detectors could be measured by comparing the output of each detector when receiving radiation from each of the sources. The output signal from each of the detectors could then be adjusted to compensate for the variation in the responsivities among the various detectors.
While the methods for providing D.C. restoration and responsivity equalization described above were somewhat effective, they required an imaging sensor to scan at least one thermal reference source during the inactive portion of its scan cycle. The imaging sensor therefore often had to be used in conjunction with relatively complex opto-mechanical mechanisms. Additional complications also existed with respect to maintaining the temperature of the thermal reference sources within the required operating limits.